Thanks David. Awesome video as always.
Is it possible to have writebook as a part of a personal website such as(www.driftingruby.com/writebook) hosted on heroku or render?

Yes, you could. You would basically need to have a proxy where any requests to /writebook would be forwarded to a different ip/port/site/etc. You could probably do this at a CloudFlare level with a page rule that matches /writebook/* to forward to a different address. Typically with Apache or Nginx, you would use a proxy pass through to another location.